1. Field of the Invention
The present invention relates to a particle selection method of analyzing a mass of particles or selecting particles like atoms, molecules, ions, or ultrafine particles (clusters) with a high resolution, and a time-of-flight mass spectrometer as a particle selection apparatus.
2. Description of Related Art
FIG. 1 shows an example of a conventional time-of-flight mass spectrometer. As shown in FIG. 1, the mass spectrometer has an ionization laser 1, an accelerator 2, a deflector 3, a reflector 4, and an ion detector 5. When measuring a mass spectrum of molecules, ultrafine particles, or the like, those neutral particles P are first irradiated by laser using the ionization laser 1 and are ionized to form charged particles (ions) Pe. The charged particles Pe are accelerated only over a constant distance by a static electric field between electrodes 2a and 2b of the accelerator 2 and then are deflected by the deflector 3 in a predetermined manner. Then they are reflected by the static electric field in the reflector 4 to be impinged on the ion detector 5. This method cannot exclude the charged particles with the mass or charge state changed during the acceleration or during the flight. The charged particles Pe of different masses do not pass the same position at the same time after the acceleration.
In the process of the ionization, for example, by the ionization laser 1, molecules or ultrafine particles existing within a finite region become charged particles, so that their initial positions are distributed. Since molecules or ultrafine particles are introduced to the ionization region as a neutral beam, they already have initial velocities before the acceleration by the electric field, and moreover those initial velocities are distributed. Those distributions of the initial positions and the initial velocities decrease mass resolution remarkably. Therefore in general, the conventional methods adopt a two-stage acceleration method or a two-stage reflector, but those methods cannot remove perfectly the factors which decrease those mass resolution.
In the process of the ionization, internal energy of the charged particles increase and the some charged particles decay by fission into charged fragments or dissociate while emitting or evaporating a part of their particles. The time-of-flight of the charged particle which have decayed or dissociated after the instance of the ionization is different from those of the parent particles and those of particles with the same mass as the fragmented particles, resulting in a significant decrease in mass resolution. The reflector 4 of the conventional time-of-flight mass spectrometer can reduce the influence in same degree, but not perfectly.
In a conventional time-of-flight mass spectrometer, charged particles with different masses do not pass the same position at the same time after the starting of the acceleration. Thus, only charged particles of a particular mass is subject to the laser irradiation in small region after the ionization. In addition, a conventional time-of-flight mass spectrometer cannot measure only charged particles with prescribed charge state.